1. Field of the Invention
This invention relates to thermodynamic engine systems for generating the mechanical power for diverse utilization devices. More particularly, this invention relates to thermodynamic engine systems which utilize the temperature differential between two fluids to provide mechanical energy to follow-on machinery.
2. Description of the Prior Art
Thermodynamic engine systems are known in which mechanical or other forms of energy are obtained from the thermal differential between a primary working fluid and a secondary fluid, the latter typically comprising ambient air. In a typical system of this type, rotary mechanical power is obtained from the temperature differential between a primary working fluid such as nitrogen stored at cryogenic temperatures and ambient air by conducting the stored primary fluid through successive energy conversion stages in each of which the thermal-potential energy is converted into rotary mechanical energy. This rotary mechanical energy is then used directly to power a utilization device, such as a pump. Alternatively, the rotary mechanical energy may be converted in a known way to reciprocating mechanical energy for driving suitable follow-on devices.
Each engine stage in such a thermodynamic engine system typically comprises a heat exchanger in which the primary working fluid is heated at constant pressure to approximately ambient temperature and an expansion engine in which the heated fluid from the outlet of the associated heat exchanger is permitted to expand to produce mechanical energy.
Thermodynamic engine systems of this type operate with little or no noise pollution and, since the only exhaust product is an inert gas such as nitrogen, contribute no chemical pollution to the ambient atmosphere, and are thus highly desirable from an ecological standpoint. However, while some engines have been found suitable for limited applications, in general known engine systems of this type suffer from the disadvantage of being relatively inefficient.
One factor contributing to the relative inefficiency of known thermodynamic engine systems is the accumulation of ice on the heat exchangers. As the working fluid is heated and passed through each of the heat exchangers, the ambient air circulating past the external heat exchanger surfaces is cooled accordingly, causing the moisture in the air to freeze. As this moisture freezes, ice forms on the exchanger external surfaces. The resulting accumulation of ice impairs the thermal transfer efficiency of the heat exchangers, which decreases the overall efficiency of the thermodynamic engine system accordingly. With prolonged operation, this ice accumulation ultimately renders the system completely inoperative.
Attempts have been made to allieviate the problem of heat exchanger ice accumulation. For example, in copending patent application Ser. No. 182,994, filed Sept. 23, 1971 for "Nitrogen Vapor Engine" now U.S. Pat. No. 3,786,631 issued Jan. 24, 1974, a thermodynamic engine system is disclosed in which ice accumulation on the thermal transfer surfaces of each engine stage heat exchanger is minimized by means of a mechanical arrangement for abrading ice formed thereon. While this arrangement has proven satisfactory for some applications, it suffers from the disadvantage of requiring substantial amounts of mechanical energy to operate, which energy must be obtained from the available mechanical energy supplied by the expansion engine. Diversion of any amount of mechanical energy available at the output of this system is, of course, undesirable since the next available power is reduced accordingly.
In addition to the ice accumulation problem, other facts are known which contribute to the relative inefficiency of known thermodynamic engine systems. An important one of such factors is the constant pressure heat exchange portion of the thermodynamic cycle typically employed in known systems. When the working fluid is cycled along a given isobar in passing through a constant pressure heat exchanger, the entropy of the fluid irreversibly increases at a substantial rate. This irreversible increase reduces the total number of effective fluid work cycles which can be performed before the working fluid is exhausted. Since known systems typically embody several constant pressure heat exchangers, the total number of effective work cycles is severely reduced. Attempts to solve this problem have not met with wide success.